Tibia part of a new joint endoprosthesis

ABSTRACT

A tibia part of a knee joint prosthesis includes a tibia platform having a distal side, and an anchoring element. The anchoring element is arranged on the distal side of the tibia platform and attached to the tibia platform. In one embodiment, the anchoring element is formed by a shield having a height which is smaller than a width of the shield, and the shield has a U-, C- or V-shape as seen from the tibia platform.

FIELD OF THE INVENTION

The invention relates to a tibia part of a new joint endoprosthesis witha tibia platform or tibia plateau and with an anchoring element which isarranged on the distal side of the tibia platform and which may bescrewed to the tibia platform.

BACKGROUND OF THE INVENTION

In a cement free knee joint replacement, additional stabilisation of atibia platform by means of Spongiosa screws is the state of the art. Inaddition to the screws, often pegs, webs or ribs which are connected tothe plateau, are common. The advantage of screws lies in the fact that,as a result of the initial stress produced thereby, a compression forceis produced between the tibia platform and the bone during implantation.As a result, the primary stability is increased. Furthermore, bone isencouraged to grow into the distal or rear surface of the tibiaplatform. However, anchoring with screws has the disadvantage that thelatter can be displaced backwards if osteonecroses or osteolyses occurunder the tibia platform. Furthermore, the danger exists in thepreviously mentioned construction that detached polyethylene particleswhich originate from the polyethylene inlay mounted on the proximal sideare displaced through the screw holes formed in the tibia platform intothe bone. This then leads to local osteolyses.

In the relevant literature, there are described various cylindricalexpansion anchors on the distal side of the tibia platform. These havethe advantage that they make a pretension force possible. However, sincethey are based on the expansion principle there is no assurance that theanchor remains securely connected to the tibia platform. The possibilityexists of being displaced and hence the formation of a gap betweenexpansion anchor and tibia plateau. Polyethylene particles, coming fromthe mentioned inlay, may be displaced through this gap into the bone andlead to osteolyses. In addition, the point shaped support which isproduced by anchors is disadvantageous for a long term osteointegrationof the implant.

As an alternative to short anchors, a long intramedullar support can beused. This support can be completed by ribs by means of which a somewhatmore widespread supporting of the tibia platform is obtained. Aconstruction of this type is known e.g. from U.S. Pat. No. 4,938,769.The use of an intramedullar support has the disadvantage however thatthe load transference takes place in the distal region. The proximalbone beneath the tibia platform is in contrast only slightly stressed.This effect, so called “stress shielding” leads to bone resorption andin the long term to loosening of the implant.

SUMMARY OF THE INVENTION

It would therefore be of advantage if by means of an appropriateconstruction, on the one hand pre-tension could be produced duringimplantation and if simultaneously the tibia platform and the anchoringelement formed a unit so that, in the first place, displacing of theanchoring element is impossible and secondly a hermetic seal occursagainst the detached polyethylene particles.

An aspect of the invention involves a tibia part of a knee jointprosthesis. The tibia part comprises anchoring element and a tibiaplatform which has a distal side. The anchoring element is arranged onthe distal side of the tibia platform and attached to the tibiaplatform. Correspondingly, the anchoring element according to theinvention is formed by a shield in the form of a U-, C- or V- shape asseen from the front, the height of which is smaller than the width. Itis therefore crucial that the anchoring shield in its implantedcondition has a greater horizontal extension than vertical extension orrather height. As a result of the relatively small height of theanchoring shield the occurrence of distal load transference isprevented. At the same time, the large-surface support in the boneoffers increased resistance to horizontal transverse loads and hencetilting of the tibia platform. In order to produce a pre-stressing forceduring implantation, the anchoring shield is firstly inserted into theproximal tibia bone and indeed so far that the upper edge of theanchoring shield is sunk to a slight extent beneath the bone surface.Next, the tibia platform is screwed into the anchoring shield which issunk in the bone. As a consequence, a pre-stress force between the tibiaplatform and the bone is produced because of the friction between theanchoring shield and the bone. Since no expansion effect occurs, theshield can adjust in a proximal direction and hence be screwed to thetibia platform to be flush and free from play without the pre-stressforce being noticeably lost.

The distal edge of the anchoring shield is arc-shaped, in particularformed to be convexly arc shaped, the arc line defining an arc of acircular section, elliptical section or a parabola. By means of thisconfiguration, the anchoring shield can be inserted more easily into theproximal tibia bone. In addition, it should be pointed out that to avoida loss of a pre-stress force the wall surfaces of the anchoring shieldextend parallel to one another. In one embodiment, thickness or wallstrength of the anchoring shield decreases step wise from proximal todistal.

In order to achieve an even better anchoring in the proximal tibia bone,the anchoring shield embraces, according to a further embodiment, acentral web which is in the region of maximal height or at approximatelyhalf the width and which extends in the manner of a bisector of thelateral and medial portions of the anchoring shield. If required afurther anchoring web can be arranged diametrally thereto. However, ithas been shown that as a rule the previously mentioned central web isadequate for difficult anchoring cases. This central web extendspreferably over the entire height of the anchoring shield. However, itis also conceivable to design the central web to be somewhat shorterthan the corresponding height of the anchoring shield.

In order to secure the anchoring shield to the distal side of the tibiaplatform, the anchoring shield has at least two especially threeproximally accessible threaded borings into which screws extendingthrough corresponding borings in the tibia platform may be screwed forsecuring the tibia platform to the anchoring shield. The threadedborings are preferably arranged centrally and at the lateral and alsomedial end of the anchoring shield. For this purpose, the anchoringshield is designed peg-like at the assigned spots respectively.

For easier inserting of the anchoring shield into the tibia bone, thedistal edge of the anchoring shield is designed as a cutting edge.

A further embodiment is distinguished in that on the distal side of thetibia platform a grooved shape recess is incorporated into which theanchoring shield may be fitted by its proximal front side securely.

In order to increase the pre-stress force of the anchoring shield in thetibia bone, the surface of same is preferably roughened, especiallystructured and indeed structured in a longitudinal direction. As aconsequence, the pre-stress force is also not lost if the anchoringshield is displaced in a proximal direction when screwing down the tibiaplatform. The enlargement of the contact surface between the anchoringshield and the tibia bone is crucial.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of a tibia part which is constructed according tothe invention are explained subsequently in greater detail withreference to the enclosed drawings which show:

FIG. 1: a first embodiment of a tibia part according to the invention inperspective exploded representation;

FIG. 2: a second embodiment of a tibia part according to the inventionin perspective exploded representation;

FIG. 3: a further embodiment of a tibia part according to the inventionin perspective exploded representation.

DETAILED DESCRIPTION OF THE INVENTION

The tibia part shown in FIG. 1 is characterised with the referencenumber 10 which is placed above. It comprises a tibia platform or atibia plateau 11 and an anchoring element arranged on the distal side inthe form of an anchoring shield 12. The anchoring shield is angled in aV-shaped in front view. The height “H” is noticeably smaller than thewidth “B” in the shown embodiment, the height “H” of the anchoringshield 12 is roughly half the projected width “B”. Instead of theV-shaped angle the anchoring shield 12 can also be formed U- or C-shapedin front view.

The anchoring shield 12 is screwed to the tibia platform 11 by means ofscrews 13 which extend through borings 14 in the tibia platform 11 insuch a manner that the proximal front side of the anchoring shield abutsthe distal side of the tibia platform along the full surface.

The distal edge 15 of the anchoring shield 12 is configured arc shaped,namely convexly arc shaped. The arc line can define an arc section, anelliptical section or a parabola.

For the purpose of screwing down the anchoring shield 12 and the tibiaplatform 11, the anchoring shield 12 has three proximally accessiblethreaded borings 16 into which the screws 13 extending throughcorresponding borings 14 in the tibia platform 11 may be screwed forsecuring the tibia platform 11 to the anchoring shield 12. The screws 13are oval headed screws or flat headed screws.

The threaded borings 16 are central in the shown embodiment, given byway of example, i.e. arranged or constructed in the region of the angleof anchoring shield and at the lateral and also at the medial ends ofthe same. For this purpose, the anchoring shield is designed thickenedat these points and roughly peg-shaped. The distal edge 15 of theanchoring shield 12 may even be designed preferably as a cutting edge.

The surface of the anchoring shield is roughened, especiallylongitudinally structured. In the embodiment according to FIG. 3, whichis described in even more detail further down, the surface structuringcan also extend transversely since, in this embodiment, the anchoringshield remains immovable after being inserted into the tibia bone. Theanchoring shield also is not displaced in a proximal direction duringsecuring of the tibia platform.

The embodiment according to FIG. 2 differs from the one according toFIG. 1 in that in the region of maximal height or at roughly half thewidth the anchoring structure encompasses a central web 17 which extendsin the manner of a bisector between the lateral and medial section ofthe anchoring shield 12. Moreover this embodiment corresponds to the oneaccording to FIG. 1 so that a further description of same issuperfluous. The central web 17 is therefore arranged centrally anddirected in a dorsal direction in its implanted state. Said web 17offers additional support for the tibia platform. The posteriorbackwards displacement of the tibia platform is reduced, if theresulting joint force is displaced dorsally as occurs while bending.Furthermore, it may also be mentioned about the embodiment according toFIG. 2 that the peg-like thickenings for forming the threaded borings 16form the lateral and medial closure of the anchoring shield 12. Themiddle or central web 17 may likewise be provided at the ends with sucha peg-like thickening for forming a threaded boring.

The central web 17 extends preferably over the entire height “H” of theanchoring shield 12, as can be seen in the embodiment according to FIG.3. However, it is also conceivable to construct the height of thecentral web 17 to be somewhat reduced.

The embodiment according to FIG. 3 differs from the embodiment accordingto FIGS. 1 and 2 in that on the distal side 18 of the tibia platform 11a groove shaped recess 19 is incorporated into which the anchoringshield 12 can be fitted by its proximal front side 20 in a play- freemanner. The geometry of the groove-like recess 19 corresponds of courseto the geometry of the front side 20 of the anchoring shield 12. Thisembodiment has the advantage that the anchoring shield does not requireto be inserted into the tibia bone in a recessed manner since the gaprequired for bracing is already incorporated in the tibia platform. Thefurther advantage exists that the seal between the tibia platform andthe anchoring shield is improved.

The outer edge of the central web 17 is directed towards the anchoringshield 12 in a proximal to distal direction. As a result, the insertingof the anchoring shield 12 into the tibia bone is facilitated.

The proximal side of the tibia platform 11 is designed in theconventional manner to receive a polyethylene inlay.

The described tibia part consists of a bio-compatible material,especially titanium or titanium alloy as is normal for endoprosthetics.

The two part design of the described tibia part is comparatively simpleas far as production technique is concerned. Hence, in DE 30 13 155 C2for example a one-piece embodiment is shown and described. This isexceedingly demanding from a production technique point of view.Complicated shapes are required. Reprocessing is also only possible at ahigh cost. Furthermore, this known embodiment has the disadvantagesportrayed above with reference to U.S. Pat. No. 4,938,769 of so-calledstress shielding.

It may also be pointed out that the distal edge 15, in the embodimentaccording to FIGS. 2 and 3, between the central web 17 and the lateralor medial end of the anchoring shield 12 respectively can extend in astraight line or arc concavely.

It might also be mentioned that the distal or lower side 18 of the tibiaplatform 11 is preferably provided with projections which are arrangedat a spacing from one another. The projections can have the form ofribs, cutting edges, pegs, cylinders, needles or similar. By means ofprojections of this type, growth of the bone onto the platform 11 isencouraged.

Tests have shown that a mutual spacing of the mentioned projections inthe order of 3 to 6 mm, especially 4 to 5 mm is particularlyadvantageous for growing of the bone. The height of the projections isroughly 0.5 to 2.5 mm, especially approximately 1 mm.

It might also be mentioned finally, that it is advantageous to providethe surface of the anchoring shield with a transverse structure,especially in the form of transverse ribs or transverse teeth. As aconsequence, the hold of the anchoring shield 12 on the bone can beincreased without the connection of the shield to the tibia platformbeing impeded.

The mentioned transverse structuring leads to a type of interlocking ofthe anchoring shield to the bone. As long as a longitudinal structure isprovided, the anchoring in the bone is consequently likewise increased,indeed because of the enlarged contact surface between the anchoringshield and the bone.

The entirety of features present in the application documents areclaimed to be essential to the invention insofar as they, on anindividual basis or in combination, are new relative to the state of theart.

List of Reference Numbers:

10 tibia part

11 tibia platform

12 anchoring shield

13 screws

14 boring

15 distal edge

16 threaded boring

17 central web

18 distal side

19 groove-shaped recess

20 proximal front side

What is claimed is:
 1. A tibia part of a knee joint prosthesis,comprising: a tibia platform having a distal side; and an anchoringelement arranged on the distal side of the tibia platform and attachedto the tibia platform, the anchoring element formed by a shield having aheight which is smaller than a width of the shield, the shield having anupper surface facing the distal side of the tibia platform, a shapeselected from a group consisting of a U-shape, C-shape and V-shape, anda distal edge which is convexly arc-shaped, an arc line of the distaledge defining a section selected from the group consisting of a circularsection, an elliptical section, and a parabola.
 2. The tibia part ofclaim 1, further comprising a central web positioned in an area in whichthe height of the shield is maximal, the central web bisecting theshield into a lateral portion and a medial portion.
 3. The tibia part ofclaim 2, wherein the central web extends over the entire height of theshield.
 4. The tibia part of claim 1, wherein the shield has at leasttwo proximally accessible threaded borings at the upper surface, thethreaded borings receiving screws extending through correspondingborings in the tibia platform to secure the tibia platform to theshield.
 5. The tibia part of claim 4, wherein the shield has threethreaded borings.
 6. The tibia part of claim 4, wherein the threadedborings are placed centrally on the lateral portion and the medialportion of the anchoring shield.
 7. The tibia part of claim 1, wherein adistal edge of the shield is designed as a cutting edge.
 8. The tibiapart of claim 1, wherein the anchoring shield is connected to the distalside of the tibia platform along the upper surface and without play. 9.The tibia part of claim 1, further comprising a recess on the distalside of the tibia platform, the recess configured to receive a proximalfront side of the shield without play.
 10. The tibia part of claim 1,wherein a shield surface is roughened or structured.
 11. The tibia partof claim 1, wherein the distal side of the tibia platform comprisesprojections which are arranged at a spacing from one another sufficientto encourage growth of bone onto the tibia platform.
 12. The tibia partof claim 11, wherein the projections have a shape selected from a groupconsisting of ribs, cutting edges, pegs, needles, and cylinders.
 13. Thetibia part of claim 11, wherein the projections on the distal side ofthe tibia platform have a spacing of approximately 3-6 mm.
 14. The tibiapart of claim 11, wherein the projections have a spacing ofapproximately 4-5 mm.
 15. The tibia part of claim 10, wherein the shieldsurface has a structure that has at least one of a longitudinal andtransverse orientation.
 16. The tibia part of claim 15, wherein theshield surface comprises teeth which have at least one of a longitudinaland transverse orientation.
 17. A tibia part of a knee joint prosthesis,comprising: a tibia platform having a distal side; and an anchoringelement arranged on the distal side of the tibia platform and attachedto the tibia platform, the anchoring element formed by a shield having aheight which is smaller than a width of the shield, the shield having aproximal edge facing the distal side of the tibia platform, said shieldhaving a distal edge configured for insertion into bone, wherein saidshield decreases in thickness in a stepwise manner from said proximaledge to said distal edge such that opposing wall surfaces of the shieldremain parallel at each step.
 18. A method of installing a knee jointprosthesis, said prosthesis comprising a tibia platform and an anchoringelement, said method comprising: inserting said anchoring element into aproximal portion of a tibia bone until an upper surface of saidanchoring element is sunk beneath the tibia bone surface; and securingsaid tibia part to said anchoring element.
 19. The method of claim 18,wherein said anchoring element comprises a shield having a height whichis less than a width of said shield.